A conventional spark plug typically has a metal shell that is fixed by caulking to an outer circumference of a ceramic insulator that holds a center electrode therein (See, for example, Japanese Patent Application Laid-Open (kokai) No. 11-345676). Such metal shell of the spark plug has a pair of flanges projecting in an outer circumference direction. During a “caulking process,” a cylindrical groove, bulging out in the outer circumference direction, is formed between the flanges. Examples of the flanges in the metal shell are a tool engagement portion assuming a polygonal-shape for engaging with a tool for mounting a spark plug to an engine head, or a trunk portion for compressing a gasket towards an engine head.
Recently, a reduction in size of a spark plug is considered as one of the various resolutions of fuel efficiency improvement in an internal combustion engine and of emission gas reduction. However, deterioration in strength of the metal shell has not been considered in connection with the miniaturization of the spark plug. For example, when the metal shell is miniaturized with the same reduction ratio as that of the spark plug, the strength of a groove in the metal shell could not fully be achieved, which may lead to a crack due to impact or stress corrosion.
In light of the above-described problems, an object of the present invention is to provide a technique capable of reducing the size of a spark plug while securing the integrity of the groove in the metal shell.